Multiple Video Cards

These are technologies that enable two graphics cards to work in concert to render a single scene. Since 3D graphics is a highly parallelizable computing problem, adding more GPUs can increase performance substantially.

The old 3dfx was actually the first company to support multiple graphics cards. 3dfx invented SLIscan line interleavewhich allowed two Voodoo2 cards to work together. nVidia was the first company out of the chute to support dual PCI Express graphics cards. nVidia's version of SLIwhich the company spelled out "scalable link interface"required you to install two cards with identical GPUs. Currently, SLI works only in motherboards using nVidia chipsets. The two cards must synchronize the data in the scene being rendered. Most nVidia cards add an additional connector between the two cards to help facilitate the data synchronization.

ATI has recently started shipping its CrossFire dual-graphics-card technology. Unlike SLI, CrossFire communicates via the DVI port; you daisy-chain two cards via a DVI pass-through cable. Also, CrossFire requires that one of the cards be a special "CrossFire Edition" card, which has an extra chip on board that acts as a compositing engine to combine the different parts of the scene being rendered by each card.

Both nVidia's technology and ATI's can synchronize the two cards' data in one of two ways. One method, known as scissoring or split-frame rendering, divides the frame into two parts; one card renders the top part, the other the bottom. In the otherAFR, or alternate frame renderingone card renders the current frame while the other renders the next frame. ATI can also use a method, called "supertiling," which splits the scene into many small chunks and divides them between the cards.

Dell recently announced it will ship a system, the XPS Renegade gaming PC, that supports quadSLIfour nVidia GPUs working together. (See the story at http://go.pcmag.com/quadsli.)

By the way, dual graphics cards can also be used without SLI or CrossFire to drive multiple displays. Also, SLI or CrossFire can be used by professional 3D artists to help speed up previews of work in progressthese technologies are not just for gamers.